IMHO, proper orthopedic diagnosis is very important for proper treatment. Check out the physical therapy sites for the knee stabilization exercises. Make sure the toes, ankles, knees, and hips stay aligned and not over extended beyond the toes. Warm up slowly. Wear knee warmers.
At 51, I have bad knees and have the train wisely. But, I still train regularly and hard.
Nidan Tenshinkai Aikido
Lucaylucay Kali JKD